Serotonin has known roles in formation of memories of cues associated with abused drugs, and in modulating the behavioral effects induced by these drugs20. To distinguish between the contributions of dopamine and serotonin neurons that express Ddc-GAL4, we tested conditioned preference in flies that had been fed the TH inhibitor 3-iodo-tyrosine (3IY) or the serotonin synthesis inhibitor alpha-methoxytryptophan (∝MTP). As expected, 3IY and ∝MTP substantially reduced dopamine and serotonin levels, respectively, in the flies’ brain (Supplementary Figs. 4c,d). Neither drug had an effect on conditioned aversion (Fig. 3c). However, 3IY, but not ∝MTP, blocked the formation of conditioned preference (Fig. 3d), thus supporting a role for dopamine in the memory of the attractive properties of ethanol.
